1. Field of the Invention
The present invention relates to a control apparatus and a control method for controlling an X-ray irradiation area.
2. Description of the Related Art
Recently, in the field of X-ray imaging for medical use, with the progress of digital technology, digital X-ray imaging apparatuses using various methods have spread. An example of such an apparatus is a computed radiography (CR) apparatus, which forms a latent image of an X-ray intensity distribution on a photo-stimulable phosphor, excites the latent image by laser scanning this photo-stimulable phosphor, and reads the generated fluorescence by a photomultiplier tube.
Further, digital X-ray imaging apparatuses have been developed which directly digitize the X-ray image without going through an optical system, by using a flat panel detector (FPD). The FPD is an X-ray flat detection device in which a phosphor is closely attached to a large surface area amorphous silicon (a-Si) sensor. In addition, FPDs have also been developed which convert X-rays into electrons by direct photoelectric conversion using amorphous selenium (a-Se), gallium arsenide (GaAs), cadmium telluride (CdTe), lead iodide (PbI2), and mercury iodide (HgI2).
However, in a digital X-ray imaging apparatus which uses such an FPD, calibration typically is performed to correct the characteristics of the sensor due to unevenness in the sensitivity of each photoelectric conversion element and unevenness in the gain in the read circuit (hereinafter referred to as “gain correction”).
The term calibration refers to the acquisition of correction data by irradiating the whole sensor surface roughly with uniform X-rays and performing imaging (this correction data is hereinafter referred to as “calibration data”). Further, the gain correction is performed by dividing (or logarithmically converting and then subtracting) the calibration data by the actually captured image of a subject (this image is hereinafter referred to as “captured image”).
However, in the above calibration, when appropriate calibration data is not acquired, the gain correction may not be correctly performed. For example, when the X-ray irradiation area is limited during calibration to an area which is narrower than the whole sensor surface, at some of the calibration data areas, data which is roughly uniformly irradiated with X-rays cannot be obtained. Therefore, the gain correction cannot be correctly performed at some of the areas of the corresponding captured image. Further, when some kind of foreign substance is present between the X-ray tube and the sensor during calibration, a foreign substance shadow is included in the calibration data. Consequently, the gain correction similarly cannot be correctly performed at some of the areas of the captured image.
Various proposals have been made as a method for resolving such issues. For example, Japanese Patent Application Laid-Open No. 2000-070261 discusses a method which detects the X-ray irradiation area from the calibration data, and issues a warning when the whole sensor surface is not irradiated with the X-rays. In this method, the fact that the X-ray irradiation area is not appropriate can be clearly notified to an operator by issuing the warning, and an operator is prompted to acquire appropriate calibration data.
Japanese Patent Application Laid-Open No. 63-181742 discusses a method which detects the position of a head during calibration, and automatically retracts the head when the head is between the X-ray tube and the sensor. In this method, the inclusion of the shadow of the head, which is a foreign substance, in the calibration data can be avoided, and appropriate calibration data can be acquired.
Japanese Patent Application Laid-Open No. 2001-351091 discusses a method which performs capturing images a plurality of times during calibration, checks the dose, irradiation area, and whether a foreign substance is included from the plurality of acquired data, and notifies the operator of those results. In this method, the dose, irradiation area, and whether there is no inclusion of foreign substances can be clearly notified to the operator, and the operator can be prompted to acquire appropriate calibration data.
When performing the above calibration, there are cases where for some reason the whole sensor surface cannot be irradiated with the X-rays. For example, a sufficient distance may not be obtained between the X-ray tube and the sensor, or due to design restrictions, a foreign substance which is present between the X-ray tube and the sensor may shield the periphery of the sensor.
In such cases, it is difficult to acquire the appropriate calibration data. Consequently, the operator may start imaging as is even if there was a warning. Thus, when imaging the subject under these conditions, a non-effective area is present where gain correction is not correctly performed at some areas of the captured image. However, in the conventional methods, there is no method which allows the operator to confirm beforehand such a non-effective area in the captured image. Therefore, imaging may be performed while the operator does not realize that the area of interest used for diagnosis is included in this non-effective area. Further, from the standpoint of protecting the subject from exposure, imaging the subject by irradiating the whole sensor surface with X-rays although the non-effective area is present, may be an issue.